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Cutting back on pesticides to boost earthworm populations in agricultural soils

Soil fauna and pesticides are not necessarily a match made in heaven. Researchers at INRA Versailles-Grignon have found that reducing pesticides by 50% leads to a significant increase in earthworm populations, especially those that live near the soil surface. This is worth keeping in mind when developing the agricultural practices of tomorrow.

Counting earthworm populations: one of the biological components measured in each agricultural system of the SIC experiment (Innovative systems under constraints) in Grignon. © INRA
By Elodie Dincuff, translated by Inge Laino
Updated on 09/01/2014
Published on 07/22/2014

Intensive farming has resulted in a loss of biodiversity in agricultural areas, but the potential benefits of organic or low-input farming for biodiversity, and in particular for soil fauna, are not well known. Earthworms, which ensure important agro-ecological functions, may be affected by the use of pesticides.

Researchers at INRA Versailles-Grignon and their colleagues have turned their attention to this issue.

Different species of earthworms in agricultural soils

Research has focused on three species of earthworms present in cultivated soils:

  • Lumbricus castaneus, present especially in top layers of soil;
  • Allolobophora chlorotica, present in the first few centimetres of soil, where it feeds;
  • Lumbricus terrestris, present at greater depths, but feeding at the surface.

Scientists monitored the populations of these three species in 30 agricultural plots near Paris from 2005 to 2012. In half of these plots, traditional farming methods were used, while organic farming was practiced in the other half. All plots were ploughed regularly, using the same ploughing techniques.  Samples were taken while the plots were used for growing winter wheat. Population counts for A. chlorotica, L. castaneus and L. terrestris reached up to 135, 105 and 44 worms respectively, per square metre.

The use of pesticides was evaluated by calculating the Treatment Frequency Index (TFI) for each plot, for all the plots together, and according to type of pesticide (fungicide, herbicide, insecticide). The annual average TFI for plots where traditional farming methods were used was 4.1.

What is the Treatment Frequency Index?

Treatment Frequency Index (TFI) is an indicator of how much pesticide is used.

TFI corresponds to the ratio between a prescribed dose of pesticide and the dose actually used on a given surface area of land. For example, one hectare treated with 70% of a prescribed dose has a TFI of 0.7.

Developed in Denmark in the 1980s, TFI can be calculated based on different factors – plot, crop, farm – or according to type of pesticide (eg fungicide, herbicide, insecticide, parasiticide).

When it comes to pesticides, not all worms are created equal

Using statistical analyses of data, researchers have been able to forecast changes in earthworm populations in farmland based on pesticide use.

According to their findings, a total increase in TFI of 0 to 4.5 (ie TFI of organic plots versus the overall reference TFI calculated in France in 2006, increased by 25%) would take a heavy toll on L. castaneus, with barely 5% of its population surviving. The other two species would fare better, maintaining approximately 30% of their populations.
The researchers explain the detrimental effects of pesticides on earthworms by the potential lethal consequences of frequent use, negative impact on reproduction and growth, and an exodus of populations from treated areas. However, not all species have the same reaction. L. castaneus, followed closely by L. terrestris, is more susceptible than A. chlorotica, which seems to suggest that the closer earthworms are to the surface, the more they are affected by pesticides.

Moreover, all pesticide treatments do not produce the same results: in general, insecticides are more hazardous to all three species than herbicides and fungicides.

Conversely, reducing pesticides by half (total TFI of 1.9, ie half the reference value established for France in 2006), as prescribed by the 2018 Ecophyto programme, would lead to a significant increase in earthworm populations. The population of L. Castaneus would be  4.8 times greater, that of L. terrestris and A. chlorotica 1.5 times greater. This would also come with a wealth of other benefits for agricultural land: structuration, maintenance, fertility, etc.

Now more than ever, assessing pesticide-related risks on human health and the environment is a key challenge of public policy. Cutting back on pesticides is top priority in the 2018 Ecophyto programme, which calls for, notably, a monitoring of indices. Of these, the Treatment Frequency Index (TFI) has been found to be key in helping farmers make the right choices and adopt the best practices for their soils and for the earthworm populations living in them.

for more information

Pelosi C. et al. 2013. Reduction of pesticide use can increase earthworm populations in wheat crops in a European temperate region. Agr. Ecosys. Environ. 181: 223.
DOI: 10.1016/j.agee.2013.10.003